Kinetics and mechanism study of low-temperature selective catalytic reduction of NO with urea supported on pitch-based spherical activated carbon

The kinetics and mechanism of selective catalytic reduction (SCR) of NO with urea supported on pitch-based spherical activated carbons (PSACs) were studied at low temperatures. NO oxidation to NO₂ catalyzed by the 0.5-0.8 nm micropores in PSACs was found to be the rate-limiting step in urea-SCR reaction, which was confirmed by both the apparent activation energy calculations and the kinetics results of urea-SCR reaction and NO oxidation on PSAC. These two reactions gave very similar negative apparent activation energies (-16.5 kJ/mol for urea-SCR reaction and -15.2 kJ/mol for NO oxidation), indicating that the adsorption of reactants on PSAC is of key importance in these two reactions. Moreover, these two reactions were both approximately first-order with respect to NO and one-half order with respect to O₂. It was found that NO₃ from the disproportionation of the produced NO₂ was quickly reduced by supported urea into N₂. After the complete consumption of supported urea, NO₂ started to release, and the carbon surface was gradually oxidized by adsorbed NO_x species. NO₃ was found to be stably adsorbed on the oxidized carbon surface. On the basis of the results obtained, a reaction mechanism of low-temperature urea-SCR reaction on PSAC was proposed and discussed.